How can I create a wall with random bumps?
If you're ok with using a shader, something like this can be accomplished by a single Noise Texture, run through a ColorRamp to give a harsh cutoff to the tops which forms the flat sections.
This is just a simple example - if you want more variation in size, you can combine different noise textures of different scales until it's more to your liking.
Remember to turn on ambient occlusion in the render properties panel.